This invention relates in general to the manufacture of a color television picture tube having an electron gun mounted in a neck of the tube, and in particular to a method of spot-knocking the electron gun. Conventionally a color television picture tube has a glass bulb including a funnel, a faceplate sealed to the flared end of the funnel and an electron gun mounted in the funnel neck for providing a source of cathode rays. The electrode gun for a color television picture tube typically has a heater and cathode, a control grid G1, a screen grid G2, a focus electrode G3, and a high voltage elecrode G4 for each of the three color beams generated by the tube.
A color television picture tube may operate with a potential difference of the order of tens of kilovolts among its electrodes. In a tri-beam color tube the three electron gun structures are positioned in the neck of the tube in closely spaced relation and the beams from the guns are deflected by a common deflection field. The close spacing and high voltages cause color television picture tubes to be particularly susceptible to arcing which can make noise that is disturbing to the user of the tube or can cause current flow in the tube and associated circuitry which may damage the tube and/or television receiver.
Such arcing has been found to be largely a function of foreign matter or contamination within the tube, and of mechanical imperfections in the construction of electron guns in the tube. Some improvement in arcing characteristics can be achieved by careful cleaning of the gun parts before they are assembled. Additionally, it is helpful to employ a high voltage spot-knocking to melt metal projections on the guns or stray particles or foreign matter within the tube after the tube has been evacuated and sealed, so that these no longer tend to promote arcing.
In one form of spot-knocking, the cathode, the heater and the low voltage electrodes G1, G2 and G3 are grounded and a pulsed positive voltage, which peaks at about 200% of the normal ultor voltage, is applied to the high voltage electrode G4 for a predetermined amount of time to burn off the loose particles or the metal projections which may reside between the electrodes.
A unique electron gun is disclosed in the referent copending application. This disclosure depicts as electron gun having a heater and cathode, a control grid G1, a screen grid G2, and four main focus lens electrodes G3, G4, G5 and G6. The unique electron gun is characterized by having small gap sizes between the grids and electrodes as compared to a conventional electron gun. In addition, these small gap sizes have substantially larger potentials applied to them than in the conventional electron gun. Due to the small gap sizes and the high potentials applied to the grids and electrodes, it is necessary to spot-knock not only the G5-G6 gap, but also the G4-G5 gap and the G2-G3 gap. For this electron gun conventional spot-knocking techniques are impractical.
This invention is most advantageously applied to an electron gun of the character described and claimed in the referent copending application.